1. Field of the Invention
The invention relates to wireless communication device and more particularly to a wireless communication device having separated groundings.
2. Description of the Related Art
For system-level layout design, good grounding is important. Generally, in order to suppress noise, the simplest and most effective way is to separate groundings with different properties, for example, groundings for analog and digital portions of circuitry. On the other hand, for signal trace routing, in order to avoid being influenced by EMI (Electromagnetic Interference), a differential architecture is a commonly used structure to improve EMS (Electromagnetic Susceptibility) of a system. For antenna design engineers, enhancing the efficiency of antennas is an important task. However, too much radiation power sometimes causes trouble to the system. For example, enhancing the antenna efficiency may result in enhancing EMI noise owing to radiation or conduction.
FIG. 1(a) shows a top view of a wireless communication device 10. An RF (Radio Frequency) grounding 120 and an analog grounding 130 are provided on a printed circuit board 100. There's a moat 110 between the RF grounding 120 and the analog grounding 130, so the RF grounding 120 and the analog grounding 130 are separated groundings. FIG. 1(a) only shows a portion of the wireless communication device 10. An on-board antenna, such as a printed inverted-F antenna 140, is provided on the printed circuit board 100 and coupled to the RF grounding 120. FIG. 1(b) is a partial enlarged view of FIG. 1(a). Differential pair routings 150 and 160 used to transmit signals are wiring devices with two traces. Parts of the differential pair routings 150 and 160 are in the same layer as the RF grounding 120 and the analog grounding 130. Other parts of the differential pair routings 150 and 160 enter other layers by drilling so as to connect with circuits or other modules on the other layers, such as an RF module. The on-board antenna on the circuit board induces strong edge current. When increasing the efficiency of antennas, the current density becomes larger on the groundings. The current energy, especially the current energy along the edges of groundings, couples the conductive EMI energy to the differential pair routings 150 and 160 via stray capacitance between groundings and differential pair routings. In the differential pair routing 150, the trace 150-a, which is closer to the antenna 140, has more EMI noise than the other trace 150-b, and thus magnitudes of EMI noise on two traces of the differential pair routing 150 are unequal. A colored figure in FIG. 5 is a simulation diagram of current density of the wireless communication device in FIG. 1(a) and FIG. 1(b). The left part of the colored figure in FIG. 5 corresponds to the structure in FIG. 1(a) and the right part of the colored figure in FIG. 5 corresponds to the structure in FIG. 1(b). As shown in right part of the colored figure in FIG. 5, red parts representing higher current density are drawn near the trace 150-a, which is closer to the antenna 140 in the differential pair routing. Orange parts representing second high current density are drawn near the other trace 150-b. Therefore, higher current density is distributed in the trace 150-a, which is closer to the antenna 140 in the differential pair routing, than in the other trace 150-b. Furthermore, as shown in FIG. 2, simulated voltage response between trances of the differential pair routing has bigger fluctuations, and hence the EMI noise can't be diminished effectively through differential pair routings.